The present invention relates to a process for the continuous preparation of nitrobenzene by nitration of benzene with mixed acid. In this process, the pressure upstream of the nitration reactor is from 14 bar to 40 bar above the pressure in the gas phase of a phase separation apparatus for separating crude nitrobenzene and waste acid.
The continuous process for the preparation of nitrobenzene of the present invention is based upon the concept of adiabatic nitration of benzene with a mixture of sulfuric acid and nitric acid (so-called “mixed acid”). An adiabatic nitration process was claimed for the first time in U.S. Pat. No. 2,256,999. More current embodiments of an adiabatic nitration process are described, for example, in EP 0 436 443 B1; EP 0 771 783 B1; and U.S. Pat. No. 6,562,247 B2. Processes in which the reaction is carried out adiabatically are distinguished by the fact that no technical measures are taken to supply heat to the reaction mixture or to remove heat from the reaction mixture.
A common feature of the known adiabatic processes is that the benzene and nitric acid starting materials are reacted in the presence of a large excess of sulfuric acid. The sulfuric acid takes up the heat of reaction that is liberated and the water formed in the reaction.
In order to carry out the reaction, nitric acid and sulfuric acid are generally mixed to form the so-called mixed acid (also called nitrating acid), and benzene is metered into the mixed acid. The product obtained reacts with the nitric acid or with “nitronium ions” formed in the mixed acid substantially to form water and nitrobenzene. Benzene is used in at least the stoichiometric amount—based on the molar amount of nitric acid—but preferably in a 2% to 10% excess compared with the stoichiometrically required amount of benzene.
The most important criterion for describing the quality of an adiabatic process for the nitration of aromatic hydrocarbons is the content of undesirable by-products in the product. Such by-products are formed by repeated nitration or oxidation of the aromatic hydrocarbon or of the nitroaromatic compound. In nitration of benzene, the content of dinitrobenzene and of nitrophenols, in particular trinitrophenol (picric acid), which is rated as explosive, is always discussed.
In order to obtain nitrobenzene with particularly high selectivity, the nature of the mixed acid to be used has been specified in detail. (See, e.g., EP 0 373 966 B1; EP 0 436 443 B1; and EP 0 771 783 B1). It has been noted that the content of by-products is determined by the maximum temperature attained by the reaction mixture (EP 0 436 443 B1, column 15, I. 22-25). It is also known that a high initial conversion is advantageous for high selectivity, and that this high initial conversion may be achieved with optimal mixing at the beginning of the reaction (EP 0 771 783 B1, paragraph [0014]).
The inexpensive and efficient configuration of the initial mixing (dispersion) and the repeated mixing (re-dispersion) of aromatic compounds in the mixed acid is the subject of numerous studies. As a result, use of mixing nozzles (EP 0 373 966 B1; EP 0 771 783 B1) and specially formed static dispersing elements (EP 0 489 211 B1; EP 0 779 270 B1; EP 1 291 078 A1; and U.S. Pat. No. 6,562,247 B2) has been proposed. It is also possible to combine the two concepts.
If static mixing elements (dispersing elements) are used for the mixing, the pressure loss at these static mixing elements is critical for the quality of the mixing. The pressure upstream of the reactor must be at least equal to the sum of the pressure losses of all the dispersing elements in the reactor. Other factors may also have to be taken into account. Such factors include the static pressure of the liquid column in the reactor and the pressure in the phase separation apparatus. Within the scope of this invention, “pressure upstream of the reactor” is understood as being the absolute pressure that the liquid starting materials are under immediately before and also on entry into the reactor.
According to the teaching of the prior art, the total pressure loss over the reactor and, where appropriate, further apparatuses connected downstream of the reactor (and accordingly also the pressure upstream of the reactor) is to be kept as low as possible. See, for example, EP 1 291 078 A2, paragraph [0017]. Another example is described in EP 2 070 907 A1, where it is disclosed that an increase in the absolute pressure upstream of the reactor from 13.5 bar to 14.5 bar as a result of deposits of metal sulfates in the dispersing elements leads to a reduction of about 18% in the throughput of sulfuric acid (Example 1). The prior art therefore teaches that high pressure losses, and accordingly, high absolute pressures, upstream of the reactor are to be avoided.
An example of the mixing of aromatic compound and mixed acid by means of a suitable nozzle without static dispersing elements is found in EP 0 373 966 B1. Here, a range of from 0.689 bar to 13.79 bar is given as a suitable range for the working pressure. “Back pressure” equals counter-pressure in the nozzle, i.e., the pressure of the liquid starting material stream (aromatic compound or mixed acid) on entry into the reactor, which is equivalent to the pressure of the starting material stream in question upstream of the reactor. (p. 5, I. 12 to 13) This disclosure also teaches that, under normal conditions, a pressure higher than 11.03 bar is not expected to be necessary (p. 5, I. 15 to 16).
The possible lower limit for the pressure upstream of the reactor is additionally established by the fact that the benzene should be in liquid form at the reactor inlet under the given conditions (U.S. Pat. No. 4,091,042, column 2, lines 14 to 17). Regarding the possible upper limit, it is to be noted that, according to the prior art, the pressure loss per static dispersing element is kept as low as possible because, in order to overcome a higher pressure loss, for example, a pump having a higher power is required, which in turn leads to higher costs for the process as a whole (EP 1 291 078 A2, paragraph [0017]). Also, attempts are generally made to keep the number and thickness (stability) of the dispersing elements preferably as low as possible and thus minimize the cost of the dispersing elements which are often produced from special tantalum material. (EP 1 291 078 A1, paragraph [0018])
The pressure inside the reactor is also limited by the material used to construct the tubular reactor. Under generally conventional conditions for the adiabatic nitration of benzene at from 80° C. to 150° C. using sulfuric acid having a concentration of from 65% by mass to 80% by mass, only tantalum, Teflon and glass are permanently resistant. High-alloy steels can likewise be used, in particular when the sulfuric acid always contains a residual amount of nitric acid, becaue nitric acid has a passivating effect on the high-alloy steel. On an industrial scale, steel pipes enamelled with glass are especially used for the adiabatic nitration of benzene. Steel enamel pipe segments are to be manufactured in accordance with DIN standard 2873 of June 2002 for nominal pressure level PN10 and at most for nominal pressure level PN25. Nominal pressure level PN25 is permissible only in the case of pipe diameters up to a nominal width of DN150 (nominal pressure levels according to EN1333, nominal width according to DIN EN ISO 6708). As is known to the person skilled in the art, the permissible operating pressure is not identical to the nominal pressure level but must be calculated in view of the temperature and material being used. At higher temperatures, the permissible operating pressure is correspondingly lower due to the reduction in the permissible material parameters. In the construction of chemical installations, fittings (valves, slides, etc.) are required in addition to apparatuses and pipes, which fittings are in turn subject to their own standards. The result of these high requirements is that the skilled person building large-scale nitration installations must be concerned with keeping the pressure within the installation, particularly the pressure upstream of the reactor, low, as long as he/she does not know that a significant advantage is obtained thereby.
Although processes described in the prior art permit the preparation of a crude nitrobenzene which has a low content of by-products, i.e., from 100 ppm to 300 ppm dinitrobenzene and from 1500 ppm to 2500 ppm nitrophenols of which picric acid can account for from 10% by mass to 50% by mass of the nitrophenols, a critically important factor for industrial production, apart from the purity of the crude nitrobenzene, is that the preparation of the nitroaromatic compounds be carried out in reaction devices that are as compact as possible. This is a particular concern in view of the constantly rising demand for nitroaromatic compounds, especially for the preparation of aromatic amines and aromatic isocyanates.
An important parameter for describing the relationship between the amount of product that can be produced and the size of the reaction device is the space-time yield (STY). STY is calculated as the quotient of the amount of the target compound that can be produced per unit time and the volume of the reaction device.STY[tnitrobenzene(m3reaction space·h)]=amount produced[tnitrobenzene/h]/reaction space[m3]
In the case of the nitration of benzene, the space-time yield is calculated as the quotient of the production of nitrobenzene in metric tonnes per hour and the volume of the reaction space. The reaction space is defined as the space which begins with the first dispersion of benzene and mixed acid and within which the reaction is completed to a degree of conversion of nitric acid of at least 99%. The reaction space is arranged in a technical device for carrying out chemical reactions, the reactor. In the simplest case, the reaction space is identical with the inside volume of the reactor. In this connection, the first dispersion means the first intensive mixing of benzene and mixed acid. This generally takes place either in a mixing nozzle or in a static mixing element. Simply combining a benzene stream and a mixed acid stream in a common feed pipe leading to the reactor, without taking particular measures to intensively mix the two streams, is not regarded as the first dispersion required in the present invention.
The residence time of the reaction mixture, consisting of the aromatic compound and the mixed acid, within the reaction space is the reaction time.
A high space-time yield is advantageous for the industrial application of a process because it makes it possible to construct compact reaction devices which are distinguished by a low investment volume per capacity.
With regard to the space-time yield of aromatic compound nitration, there is still a marked need for improvement over the prior art.
However, high space-time yields when carrying out a nitration adiabatically (in particular with a constant residence time in the reactor) inevitably lead to high temperature differences (adiabatic temperature jumps) between the start temperature (the temperature of the mixed starting materials before the start of the reaction, determined by calculating the combined temperature of the individual streams) and the reaction end temperature (the temperature after conversion of substantially all the nitric acid); and, as is clear from the prior art, high reaction end temperatures lead to an impairment of the selectivity. (See, e.g., EP 0 436 443 B1, column 15, I. 22-25).